Evaporators of the stacked plate type are already known which comprise a plurality of flat hollow bodies arranged in parallel and each composed of a pair of dishlike plates facing toward each other and brazed to each other along peripheral edges thereof, a refrigerant inlet header and a refrigerant outlet header which are arranged in the front-rear direction, a refrigerant turn portion disposed as spaced from the two headers, a plurality of forward refrigerant passage portions for holding the inlet header in communication with the turn portion therethrough, and a plurality of backward refrigerant passage portions for holding the outlet header in communication with the turn portion, the inlet header having a refrigerant inlet at one end thereof, the outlet header having a refrigerant outlet at one end thereof alongside the inlet end, a refrigerant being permitted to flow from the inlet into the inlet header, then to flow through the forward passage portions into the turn portion where the refrigerant changes its course, thereafter to flow through the backward passage portions into the outlet header so as to be sent out from the outlet, a pipe joint plate being joined to both the inlet header and the outlet header and having a refrigerant inlet portion in the form of a short tube and communicating with the inlet and a refrigerant outlet portion in the form of a short tube and communicating with the outlet, a refrigerant inlet pipe having an end inserted in and joined to the inlet portion a refrigerant outlet pipe being larger than the inlet pipe in diameter and having a constricted end portion inserted in and joined to the outlet portion (see, for example, the publication of JP-A No. 2001-241881).
The inlet pipe of such evaporators is usually smaller than the outlet pipe in diameter. With the evaporator disclosed in the above publication, therefore, the refrigerant inflow portion is so dimensioned that the inlet pipe can be inserted thereinto without constriction, while the refrigerant outflow portion is given the same size as the inflow portion, such that the constricted end portion of the outlet pipe can be inserted into the outflow portion.
In recent years, however, it has been demanded to provide evaporators further reduced in size and weight and exhibiting higher performance. To meet such a demand, an evaporator has been proposed which comprises a refrigerant inlet header and a refrigerant outlet header which are arranged in the front-rear direction, and a refrigerant circulating passage for holding the two headers in communication with each other, the circulating passage comprising two intermediate headers and a plurality of heat exchange tubes, the inlet header being opposed to one of the intermediate headers, the outlet header being opposed to the other intermediate header, a group of heat exchange tubes arranged at a spacing and in the form of at least one row disposed between each opposed pair of headers, the heat exchange tubes in the group having opposite ends joined to the opposed pair of headers respectively, the inlet header having a refrigerant inlet at one end thereof, the outlet header having a refrigerant outlet at one end thereof alongside the inlet end, a refrigerant being permitted to flow from the inlet into the inlet header and to return to the outlet header through the circulating passage so as to be sent out of the outlet (see the publication of JP-A No. 2003-214794).
Also in the case of the evaporator disclosed in the publication of JP-A No. 2003-214794, a refrigerant inlet pipe and a refrigerant outlet pipe need to be joined to the inlet header and the outlet header, respectively, so that the evaporator has a refrigerant inlet-outlet block which is brazed to both the inlet header and the outlet header and provided with a refrigerant inflow portion communicating with the inlet and a refrigerant outflow portion communicating with the outlet. However, the inlet-outlet block has a relatively great thermal capacity, and is therefore lower in brazeability, rendering the evaporator cumbersome to fabricate.
Accordingly, it is readily conceivable to braze the pipe joint plate disclosed in the publication of JP-A No. 2001-241881 to both the inlet header and the outlet header of the evaporator disclosed in the publication of JP-A No. 2003-214794, with an end of a refrigerant inlet pipe inserted in and joined to the inlet portion of the joint plate and with a constricted end portion of a refrigerant outlet pipe, having a larger diameter than the inlet pipe, inserted in and joined to the outlet portion of the joint plate. In this case, however, the evaporator has a reduced size in the front-rear direction, and the pipe joint plate therefore needs to be smaller in length in the front-rear direction, for example, up to 50 mm. If the pipe joint plate has a reduced length, e.g., up to 50 mm, in the front-rear direction, the following problem arises since the inlet portion and the outlet portion provided on the joint plate are equal in outside diameter. The inlet portion and the outlet portion of the pipe joint plate are joined to the inlet pipe and the outlet pipe usually by high-frequency brazing using a high-frequency heating coil provided around these inlet and outlet portions, whereas if the pipe joint plate is restricted in the front-rear dimension, provision of the inlet and outlet portions of approximately the same outside diameter results in a reduced spacing between the inlet and outlet portions, presenting difficulty in fitting the high-frequency heating coil around these portions, making it impossible to automatically carry out the brazing operation and necessitating cumbersome fabrication work. Furthermore, the reduced spacing between the inlet portion and the outlet portion diminishes the area of brazing joint of the joint plate portion between the inlet and outlet portions and the inlet header or the outlet header, and a faulty joint is likely to occur. When the brazing joint is faulty, short-circuiting will occur between the inlet header and the outlet header, consequently permitting the refrigerant flowing in through the inlet pipe to flow into the outlet pipe without passing through the heat exchange tubes and failing to contribute to cooling to any extent to entail seriously impaired cooling performance.
An object of the present invention is to overcome the above problems and to provide an evaporator which can be fabricated relatively easily and wherein the short-circuiting between the inlet header and the outlet header can be prevented.